There is a known device that performs power conversion by an inverter circuit including a semiconductor switching element, supplies power to a load, and includes a plurality of protection circuits such as over current protection circuits or over voltage protection circuits (for example, see PTL 1). PTL 1 discloses a technology for performing control such that an operation is stopped by urgently stopping an operation of the semiconductor switching element of the inverter circuit at the time of a major failure such as an over current or an over voltage and by slowly decreasing a current flowing in the semiconductor switching element at the time of a minor failure such as overload. That is, the semiconductor switching element enters an open state in which the current does not flow.
Further, PTL 1 discloses a control circuit that detects a failure of the inverter circuit and stops the switching element when the failure of the inverter circuit is detected.
However, when the same method as that of PTL 1 is applied to an electrical motor control inverter circuit, the electrical motor control circuit enters the open state. Therefore, an electrical motor itself freely rotates, and thus its speed gradually decreases. Further, when the electrical motor is driven, the degree of emergency is frequently different between the case of an over current and the case of an over voltage. In PTL 1, however, stopping the operation of the switching element is performed likewise in either case.
PTL 2 discloses an electrical motor control device that includes a safety shutdown circuit in which a plurality of 3-state buffer circuits are installed between a PWM generation circuit and a gate driving circuit of an inverter. The safety shutdown circuit further includes a circuit by which a high-order device can detect whether an operation of the safety shutdown circuit itself is normal.
The 3-state buffer circuit in PTL 2 blocks a PWM signal supplied to a gate driving circuit, and thus an electrical motor enters a free-run state and gradually stops. However, a control input to the 3-state buffer is configured to be manually performed by an external motor stop switch.
PTL 3 discloses a system that stops supplying power from an inverter to an electrical motor by short-circuiting a semiconductor switching element of an upper arm or a lower arm of an inverter circuit, when a failure such as openness of a switch (contactor) supplying direct-current (DC) power from an electrical accumulator to the inverter circuit occurs during driving of an electrical vehicle.